Towards a robust concept for modelling zooplankton migration

Zooplankton drift with the water masses in the horizontal plane but may regulate their position in the vertical dimension (Murray & Hjort 1912; Russel 1927). Many of these animals show ontogenetic or seasonal differences in their vertical distribution spanning more than 1000 m (Østvedt 1955). On shorter time-scales their pattern of diel vertical migrations (DVM) may result in daily displacements of the peak distribution from surface to depths of several hundred meters – and back again (Longhurst 1976; Wiebe & al. 1979). Such migrations expose the zooplankton to water masses of variable shearvelocities over daily as well as seasonal time-scales. The net transportation effect of this may add considerable complexity to the spatial dynamics of such populations (Hardy & Gunther 1935; Hardy 1936; Frasetto & al. 1962; Kaartvedt, 1993; Eiane & al. 1997; Hill 1998). The DVM behaviour of zooplankton has been studied quite extensively over the years (Kerfoot 1985; Lampert 1989; Ohman 1990; Fiksen 1997). Today this behaviour is understood as a strategy for trading off mortality risk and energy intake over the life history of the animals (Stich & Lampert 1981; Iwasa 1982). Although the longterm optimal balancing of predation risk versus growth offers the best ultimate explanation for DVM behaviour in zooplankton (McLaren 1963; Stich & Lampert 1981; Aksnes & Giske 1990; Houston & al. 1993), we may need to invoke multiple proximate explanations or mechanisms to fully understand the adaptiveness of the trait. A brief survey of the literature suggests that the vertical behaviour of planktonic organisms is sensitive to ontogeny (Longhurst 1976; Heath 1999), light intensity (Kampa & Boden 1954; Longhurst 1976; Baliño & Aksnes 1993), photoperiod (Miller & al. 1991) food distribution (Aksnes & Giske 1990), predator abundance (Enright & Honegger 1977; Dodson 1988; Kaartvedt 1996), and the functional types of predators present (Ohman & al. 1983). The plasticity of vertical migration strategies, observed both among and within populations (Ohman & al. 1983; Heath 1999) further suggests that selection has favoured the evolution of proximate response strategies that are sufficiently functional under the range of stimuli variation encountered in nature (Ohman 1990). Resolving such complex and large-scaled processes solely by focusing on field efforts may prove time consuming and costly. Alternatively, further insight in the dynamics underlying the distribution of zooplankton in space and time may be sought by incorporating zooplankTowards a robust concept for modelling zooplankton migration